U.S. patent number 10,277,373 [Application Number 15/323,670] was granted by the patent office on 2019-04-30 for method for generating and transmitting pilot sequence using non-cazac sequence in wireless communication system.
This patent grant is currently assigned to LG ELECTRONICS INC.. The grantee listed for this patent is LG ELECTRONICS INC.. Invention is credited to Hangyu Cho, Jinsoo Choi, Jiwon Kang, Kilbom Lee, Wookbong Lee, Dongguk Lim, Eunsung Park.
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United States Patent |
10,277,373 |
Lee , et al. |
April 30, 2019 |
Method for generating and transmitting pilot sequence using
non-CAZAC sequence in wireless communication system
Abstract
Disclosed is a method for transmitting a pilot sequence, the
method comprising: generating a sequence set on the basis of a
baseline sequence having non-CAZAC properties; and selecting any
one pilot sequence form among sequence sets corresponding to
different additional information, and transmitting same to a
receiver.
Inventors: |
Lee; Kilbom (Seoul,
KR), Kang; Jiwon (Seoul, KR), Lee;
Wookbong (Seoul, KR), Choi; Jinsoo (Seoul,
KR), Lim; Dongguk (Seoul, KR), Cho;
Hangyu (Seoul, KR), Park; Eunsung (Seoul,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
LG ELECTRONICS INC. |
Seoul |
N/A |
KR |
|
|
Assignee: |
LG ELECTRONICS INC. (Seoul,
KR)
|
Family
ID: |
55351359 |
Appl.
No.: |
15/323,670 |
Filed: |
August 19, 2015 |
PCT
Filed: |
August 19, 2015 |
PCT No.: |
PCT/KR2015/008659 |
371(c)(1),(2),(4) Date: |
January 03, 2017 |
PCT
Pub. No.: |
WO2016/028082 |
PCT
Pub. Date: |
February 25, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170149540 A1 |
May 25, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62038852 |
Aug 19, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L
5/005 (20130101); H04L 27/18 (20130101); H04L
27/2613 (20130101); H04L 27/0008 (20130101); H04L
27/26 (20130101); H04L 5/0048 (20130101); H04L
5/0026 (20130101) |
Current International
Class: |
H04L
5/00 (20060101); H04L 27/18 (20060101); H04L
27/00 (20060101); H04L 27/26 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101262687 |
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Sep 2008 |
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CN |
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10-2009-0003185 |
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Jan 2009 |
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KR |
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10-2009-0026019 |
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Mar 2009 |
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KR |
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10-2011-0005748 |
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Jan 2011 |
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KR |
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WO 2007/084988 |
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Jul 2007 |
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WO |
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Other References
Kang et al., "Optimal Pilot Sequence Design Based on Chu Sequences
for Multi-cell Environments," The Journal of Korean Institute of
Communications and Information Sciences, vol. 34, No. 11, Nov.
2009, pp. 1113-1121, with an English abstract. cited by
applicant.
|
Primary Examiner: Nguyen; Hanh N
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is the National Phase of PCT International
Application No. PCT/KR2015/008659, filed on Aug. 19, 2015, which
claims priority under 35 U.S.C. 119(e) to U.S. Provisional
Application No. 62/038,852, filed on Aug. 19, 2014, all of which
are hereby expressly incorporated by reference into the present
application.
Claims
The invention claimed is:
1. A method of transmitting a signal by a transmitter to a receiver
in a wireless communication system, the method comprising:
generating a baseline sequence having non-Constant Amplitude Zero
Auto Correlation (non-CAZAC) properties; generating a plurality of
pilot sequences by shifting the baseline sequence in a time domain
based on a shifting value; and transmitting a pilot sequence of a
sequence set composed of the plurality of pilot sequences to the
receiver, wherein each of the plurality of pilot sequences are
related to different information, respectively, and wherein the
shifting value is determined such that a correlation between two
pilot sequences of the sequence set is minimized.
2. The method of claim 1, wherein a minimum value of the shifting
value is determined based on at least one of a channel effective
delay period and/or a timing offset.
3. The method of claim 1, wherein a minimum value of the shifting
value is determined based on a channel effective delay period only
or both the channel effective delay period and a timing offset,
based on a capability of estimating the timing offset by the
receiver.
4. The method of claim 1, wherein the plurality of pilot sequences
are generated by using a quantized Chu sequence as the plurality of
pilot sequences.
5. The method of claim 4, wherein the quantized Chu sequence is
generated by approximating a phase of a Chu sequence to a phase of
a predetermined constellation.
6. The method of claim 5, wherein the predetermined constellation
is one of BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase
Shift Keying), 8-PSK (Phase Shift Keying), 16-PSK and 32-PSK.
7. A transmission device for transmitting a signal to a reception
device in a wireless communication system, the transmission device
comprising: a transmitter; a receiver; and a processor connected to
the transmitter and the receiver to operate, wherein the processor
is configured to: generate a baseline sequence having non-Constant
Amplitude Zero Auto Correlation (non-CAZAC) properties, generate a
plurality of pilot sequences by shifting the baseline sequence in a
time domain based on a shifting value, and control the transmitter
to transmit a pilot sequence of a sequence set composed of the
plurality of pilot sequences to the reception device, wherein each
of the plurality of pilot sequences are related to different
information, respectively, and wherein the shifting value is
determined such that a correlation between two pilot sequences of
the sequence set is minimized.
8. The transmission device of claim 7, wherein a minimum value of
the shifting value is determined based on at least one of a channel
effective delay period and/or a timing offset.
9. The transmission device of claim 7, wherein a minimum value of
the shifting value is determined based on a channel effective delay
period only or both the channel effective delay period and a timing
offset, based on a capability of estimating the timing offset by
the reception device.
10. The transmission device of claim 7, wherein the processor
generates the plurality of pilot sequences using a quantized Chu
sequence as the plurality of pilot sequences.
11. The transmission device of claim 10, wherein the quantized Chu
sequence is generated by approximating a phase of a Chu sequence to
a phase of a predetermined constellation.
12. The transmission device of claim 11, wherein the predetermined
constellation is one of BPSK (Binary Phase Shift Keying), QPSK
(Quadrature Phase Shift Keying), 8-PSK (Phase Shift Keying), 16-PSK
and 32-PSK.
Description
TECHNICAL FIELD
The following description relates to a wireless communication
system and, more specifically, to methods and devices for
generating and transmitting a pilot sequence using a non-CAZAC
sequence in a wireless LAN system.
BACKGROUND ART
Recently, with development of information communication technology,
various wireless communication technologies have been developed.
Among others, a wireless local area network (WLAN) enables wireless
access to the Internet using a portable terminal such as a personal
digital assistant (PDA), a laptop, a portable multimedia player
(PMP) in a home, an enterprise or a specific service provision area
based on radio frequency technology.
In order to overcome limitations in communication rate which have
been pointed out as weakness of a WLAN, in recent technical
standards, a system for increasing network speed and reliability
and extending wireless network distance has been introduced. For
example, in IEEE 802.11n, multiple input and multiple output (MIMO)
technology using multiple antennas in a transmitter and a receiver
has been introduced in order to support high throughput (HT) with a
maximum data rate of 540 Mbps or more, to minimize transmission
errors, and to optimize data rate.
As next-generation communication technology, machine-to-machine
(M2M) communication technology has been discussed. Even in an IEEE
802.11 WLAN system, technical standards supporting M2M
communication have been developed as IEEE 802.11ah. In M2M
communication, a scenario in which a small amount of data is
communicated at a low rate may be considered in an environment in
which many apparatuses are present.
Communication in a WLAN system is performed in a medium shared
between all apparatuses. As in M2M communication, if the number of
apparatuses is increased, in order to reduce unnecessary power
consumption and interference, a channel access mechanism needs to
be more efficiently improved.
DISCLOSURE
Technical Problem
An object of the present invention devised to solve the problem
lies in efficient information transmission through design of a
pilot sequence.
Another object of the present invention is to maximize the
performance of a receiver even when a non-CAZAC sequence is
used.
The technical problems solved by the present invention are not
limited to the above technical problems and other technical
problems which are not described herein will become apparent to
those skilled in the art from the following description.
Technical Solution
In an aspect of the present invention, a method of transmitting a
pilot sequence by a transmitter to a receiver in a wireless
communication system includes: generating a baseline sequence
having non-CAZAC (non-Constant Amplitude Zero Auto Correlation)
properties; generating a plurality of pilot sequences by shifting
the baseline sequence in a time domain; and transmitting a pilot
sequence selected from a sequence set composed of the plurality of
pilot sequences to the receiver, wherein the shifting of the
baseline sequence is performed in order that that a correlation
between the pilot sequences is minimized, and wherein the pilot
sequences included in the sequence set respectively correspond to
different additional information to be transmitted to the
receiver.
A minimum value of the shifting may be determined in consideration
of at least one of a channel effective delay period and a timing
offset.
When the receiver can estimate the timing offset, the minimum value
of the shifting may be determined in consideration of only the
channel effective delay period.
The generating of the plurality of pilot sequences may use a
quantized Chu sequence as the plurality of pilot sequences.
The quantized Chu sequence may be generated by approximating the
phase of a Chu sequence to a phase of a predetermined
constellation.
The predetermined constellation may be one of BPSK (Binary Phase
Shift Keying), QPSK (Quadrature Phase Shift Keying), 8-PSK (Phase
Shift Keying), 16-PSK and 32-PSK.
In another aspect of the present invention, a transmitter for
transmitting a pilot sequence to a receiver in a wireless
communication system includes: a transmission unit; a reception
unit; and a processor connected to the transmission unit and the
reception unit to operate, wherein the processor is configured to:
generate a baseline sequence having non-CAZAC properties; generate
a plurality of pilot sequences by shifting the baseline sequence in
a time domain; and transmit a pilot sequence selected from a
sequence set composed of the plurality of pilot sequences to the
receiver, wherein the shifting of the baseline sequence is
performed in order that a correlation between the pilot sequences
is minimized, and wherein the pilot sequences included in the
sequence set respectively correspond to different pieces of
additional information to be transmitted to the receiver.
Advantageous Effects
The embodiments of the present invention have the following
effects.
First, identification performance of a receiver can be improved by
defining a sequence set using a non-CAZAC sequence.
Second, the identification performance of the receiver can be
secured by minimizing a correlation between pilot sequences during
design of a sequence set.
Third, generation of problems with respect to an EVM can be
restricted using a sequence set considering the hardware
performance of a transmitter.
The effects of the present invention are not limited to the
above-described effects and other effects which are not described
herein may be derived by those skilled in the art from the
following description of the embodiments of the present invention.
That is, effects which are not intended by the present invention
may be derived by those skilled in the art from the embodiments of
the present invention.
DESCRIPTION OF DRAWINGS
The accompanying drawings, which are included to provide a further
understanding of the invention, illustrate embodiments of the
invention and together with the description serve to explain the
principle of the invention. The technical features of the present
invention are not limited to specific drawings and the features
shown in the drawings are combined to construct a new embodiment.
Reference numerals of the drawings mean structural elements.
FIG. 1 is a diagram showing an exemplary structure of an IEEE
802.11 system to which the present invention is applicable.
FIG. 2 is a diagram showing another exemplary structure of an IEEE
802.11 system to which the present invention is applicable.
FIG. 3 is a diagram showing another exemplary structure of an IEEE
802.11 system to which the present invention is applicable.
FIG. 4 is a diagram showing an exemplary structure of a WLAN
system.
FIG. 5 is a diagram illustrating a link setup process in a WLAN
system.
FIG. 6 is a diagram illustrating a backoff process.
FIG. 7 is a diagram illustrating a hidden node and an exposed
node.
FIG. 8 is a diagram illustrating request to send (RTS) and clear to
send (CTS).
FIG. 9 is a diagram illustrating power management operation.
FIGS. 10 to 12 are diagrams illustrating operation of a station
(STA) which receives a traffic indication map (TIM).
FIG. 13 is a diagram illustrating a group based association
identifier (AID).
FIGS. 14 to 16 are diagrams showing examples of operation of an STA
if a group channel access interval is set.
FIG. 17 is a diagram illustrating frame structures related to
embodiments of the present invention.
FIG. 18 is a diagram illustrating a pilot sequence.
FIG. 19 is a diagram illustrating cyclic shifting of a pilot
sequence.
FIG. 20 is a diagram illustrating a receiver structure for
identification of a pilot sequence.
FIG. 21 is a diagram illustrating signals of a sequence set
received by a receiver.
FIG. 22 is a diagram illustrating a sequence set composed of pilot
sequences generated at a predetermined interval.
FIG. 23 is a diagram illustrating received signals of the sequence
set illustrated in FIG. 22.
FIG. 24 is a diagram illustrating a timing offset.
FIG. 25 is a diagram illustrating received signals considering a
timing offset.
FIG. 26 is a diagram illustrating a procedure of controlling the
size of a zero correlation zone (ZCZ) in consideration of a timing
offset.
FIG. 27 is a diagram illustrating a pilot sequence using a CAZAC
sequence.
FIG. 28 is a diagram illustrating properties of a non-CAZAC
sequence.
FIG. 29 is a diagram illustrating a procedure of receiving a pilot
sequence using a non-CAZAC sequence.
FIG. 30 is a diagram illustrating a correlation between pilot
sequences depending on shifting values.
FIG. 31 is a diagram illustrating an embodiment using a quantized
Chu sequence.
FIG. 32 is a flowchart illustrating operation procedures of a
transmitter and a receiver according to a proposed embodiment.
FIG. 33 is a diagram illustrating configurations of a UE and a base
station related to an embodiment of the present invention.
BEST MODE
Although the terms used in the present invention are selected from
generally known and used terms, terms used herein may be varied
depending on operator's intention or customs in the art, appearance
of new technology, or the like. In addition, some of the terms
mentioned in the description of the present invention have been
selected by the applicant at his or her discretion, the detailed
meanings of which are described in relevant parts of the
description herein. Furthermore, it is required that the present
invention is understood, not simply by the actual terms used but by
the meanings of each term lying within.
The following embodiments are proposed by combining constituent
components and characteristics of the present invention according
to a predetermined format. The individual constituent components or
characteristics should be considered optional factors on the
condition that there is no additional remark. If required, the
individual constituent components or characteristics may not be
combined with other components or characteristics. In addition,
some constituent components and/or characteristics may be combined
to implement the embodiments of the present invention. The order of
operations to be disclosed in the embodiments of the present
invention may be changed. Some components or characteristics of any
embodiment may also be included in other embodiments, or may be
replaced with those of the other embodiments as necessary.
In describing the present invention, if it is determined that the
detailed description of a related known function or construction
renders the scope of the present invention unnecessarily ambiguous,
the detailed description thereof will be omitted.
In the entire specification, when a certain portion "comprises or
includes" a certain component, this indicates that the other
components are not excluded and may be further included unless
specially described otherwise. The terms "unit", "-or/er" and
"module" described in the specification indicate a unit for
processing at least one function or operation, which may be
implemented by hardware, software or a combination thereof. The
words "a or an", "one", "the" and words related thereto may be used
to include both a singular expression and a plural expression
unless the context describing the present invention (particularly,
the context of the following claims) clearly indicates
otherwise.
In this document, the embodiments of the present invention have
been described centering on a data transmission and reception
relationship between a mobile station and a base station. The base
station may mean a terminal node of a network which directly
performs communication with a mobile station. In this document, a
specific operation described as performed by the base station may
be performed by an upper node of the base station.
Namely, it is apparent that, in a network comprised of a plurality
of network nodes including a base station, various operations
performed for communication with a mobile station may be performed
by the base station, or network nodes other than the base station.
The term base station may be replaced with the terms fixed station,
Node B, eNode B (eNB), advanced base station (ABS), access point,
etc.
The term mobile station (MS) may be replaced with user equipment
(UE), subscriber station (SS), mobile subscriber station (MSS),
mobile terminal, advanced mobile station (AMS), terminal, etc.
A transmitter refers to a fixed and/or mobile node for transmitting
a data or voice service and a receiver refers to a fixed and/or
mobile node for receiving a data or voice service. Accordingly, in
uplink, a mobile station becomes a transmitter and a base station
becomes a receiver. Similarly, in downlink transmission, a mobile
station becomes a receiver and a base station becomes a
transmitter.
Communication of a device with a "cell" may mean that the device
transmit and receive a signal to and from a base station of the
cell. That is, although a device substantially transmits and
receives a signal to a specific base station, for convenience of
description, an expression "transmission and reception of a signal
to and from a cell formed by the specific base station" may be
used. Similarly, the term "macro cell" and/or "small cell" may mean
not only specific coverage but also a "macro base station
supporting the macro cell" and/or a "small cell base station
supporting the small cell".
The embodiments of the present invention can be supported by the
standard documents disclosed in any one of wireless access systems,
such as an IEEE 802.xx system, a 3rd Generation Partnership Project
(3GPP) system, a 3GPP Long Term Evolution (LTE) system, and a 3GPP2
system. That is, the steps or portions, which are not described in
order to make the technical spirit of the present invention clear,
may be supported by the above documents.
In addition, all the terms disclosed in the present document may be
described by the above standard documents. In particular, the
embodiments of the present invention may be supported by at least
one of P802.16-2004, P802.16e-2005, P802.16.1, P802.16p and
P802.16.1b documents, which are the standard documents of the IEEE
802.16 system.
Hereinafter, the preferred embodiments of the present invention
will be described with reference to the accompanying drawings. It
is to be understood that the detailed description which will be
disclosed along with the accompanying drawings is intended to
describe the exemplary embodiments of the present invention, and is
not intended to describe a unique embodiment which the present
invention can be carried out.
It should be noted that specific terms disclosed in the present
invention are proposed for convenience of description and better
understanding of the present invention, and the use of these
specific terms may be changed to another format within the
technical scope or spirit of the present invention.
1. IEEE 802.11 System Overview
1.1 Structure of WLAN System
FIG. 1 is a diagram showing an exemplary structure of an IEEE
802.11 system to which the present invention is applicable.
An IEEE 802.11 structure may be composed of a plurality of
components and a wireless local area network (WLAN) supporting
station (STA) mobility transparent to a higher layer may be
provided by interaction among the components. A basic service set
(BSS) may correspond to a basic component block in an IEEE 802.11
LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are present and each BSS
includes two STAs (STA1 and STA2 are included in BSS1 and STA3 and
STA4 are included in BSS2) as members. In FIG. 1, an ellipse
indicating the BSS indicates a coverage area in which STAs included
in the BSS maintains communication. This area may be referred to as
a basic service area (BSA). If an STA moves out of a BSA, the STA
cannot directly communicate with other STAs in the BSA.
In an IEEE 802.11 LAN, a BSS is basically an independent BSS
(IBSS). For example, the IBSS may have only two STAs. In addition,
the simplest BSS (BSS1 or BSS2) of FIG. 1, in which other
components are omitted, may correspond to a representative example
of the IBSS. Such a configuration is possible when STAs can
directly perform communication. In addition, such a LAN is not
configured in advance but may be configured if a LAN is necessary.
This LAN may also be referred to as an ad-hoc network.
If an STA is turned on or off or if an STA enters or moves out of a
BSS, the membership of the STA in the BSS may be dynamically
changed. An STA may join a BSS using a synchronization process in
order to become a member of the BSS. In order to access all
services of a BSS based structure, an STA should be associated with
the BSS. Such association may be dynamically set and may include
use of a distribution system service (DSS).
FIG. 2 is a diagram showing another exemplary structure of an IEEE
802.11 system to which the present invention is applicable. In FIG.
2, a distribution system (DS), a distribution system medium (DSM)
and an access point (AP) are added to the structure of FIG. 1.
In a LAN, a direct station-to-station distance may be restricted by
PHY performance. Although such distance restriction may be
possible, communication between stations located at a longer
distance may be necessary. In order to support extended coverage, a
DS may be configured.
The DS means a structure in which BSSs are mutually connected. More
specifically, the BSSs are not independently present as shown in
FIG. 1 but the BSS may be present as an extended component of a
network including a plurality of BSSs.
The DS is a logical concept and may be specified by characteristics
of the DSM. In IEEE 802.11 standards, a wireless medium (WM) and a
DSM are logically distinguished. Logical media are used for
different purposes and are used by different components. In IEEE
802.11 standards, such media are not restricted to the same or
different media. Since plural media are logically different, an
IEEE 802.11 LAN structure (a DS structure or another network
structure) may be flexible. That is, the IEEE 802.11 LAN structure
may be variously implemented and a LAN structure may be
independently specified by physical properties of each
implementation.
The DS provides seamless integration of a plurality of BSSs and
provides logical services necessary to treat an address to a
destination so as to support a mobile apparatus.
The AP means an entity which enables associated STAs to access the
DS via the WM and has STA functionality. Data transfer between the
BSS and the DS may be performed via the AP. For example, STA2 and
STA3 shown in FIG. 2 have STA functionality and provide a function
enabling associated STAs (STA1 and STA4) to access the DS. In
addition, since all APs correspond to STAs, all APs may be
addressable entities. An address used by the AP for communication
on the WM and an address used by the AP for communication on the
DSM may not be equal.
Data transmitted from one of STAs associated with the AP to the STA
address of the AP may always be received by an uncontrolled port
and processed by an IEEE 802.1X port access entity. In addition, if
a controlled port is authenticated, transmission data (or frames)
may be transmitted to the DS.
FIG. 3 is a diagram showing another exemplary structure of an IEEE
802.11 system to which the present invention is applicable. In FIG.
3, an extended service set (ESS) for providing wide coverage is
added to the structure of FIG. 2.
A wireless network having an arbitrary size and complexity may be
composed of a DS and BSSs. In an IEEE 802.11 system, such a network
is referred to as an ESS network. The ESS may correspond to a set
of BSSs connected to one DS. However, the ESS does not include the
DS. The ESS network appears as an IBSS network at a logical link
control (LLC) layer. STAs included in the ESS may communicate with
each other and mobile STAs may move from one BSS to another BSS
(within the same ESS) transparently to the LLC layer.
In IEEE 802.11, relative physical locations of the BSSs in FIG. 3
are not assumed and may be defined as follows. The BSSs may
partially overlap in order to provide consecutive coverage. In
addition, the BSSs may not be physically connected and a distance
between BSSs is not logically restricted. In addition, the BSSs may
be physically located at the same location in order to provide
redundancy. In addition, one (or more) IBSS or ESS network may be
physically present in the same space as one (or more) ESS network.
This corresponds to an ESS network type such as a case in which an
ad-hoc network operates at a location where the ESS network is
present, a case in which IEEE 802.11 networks physically overlapped
by different organizations are configured or a case in which two or
more different access and security policies are necessary at the
same location.
FIG. 4 is a diagram showing an exemplary structure of a WLAN
system. FIG. 4 shows an example of an infrastructure BSS including
a DS.
In the example of FIG. 4, BSS1 and BSS2 configure an ESS. In the
WLAN system, an STA operates according to a MAC/PHY rule of IEEE
802.11. The STA includes an AP STA and a non-AP STA. The non-AP STA
corresponds to an apparatus directly handled by a user, such as a
laptop or a mobile phone. In the example of FIG. 4, STA1, STA3 and
STA4 correspond to the non-AP STA and STA2 and STA5 correspond to
the AP STA.
In the following description, the non-AP STA may be referred to as
a terminal, a wireless transmit/receive unit (WTRU), a user
equipment (UE), a mobile station (MS), a mobile terminal or a
mobile subscriber station (MSS). In addition, the AP may correspond
to a base station (BS), a Node-B, an evolved Node-B (eNB), a base
transceiver system (BTS) or a femto BS.
1.2 Link Setup Process
FIG. 5 is a diagram illustrating a general link setup process.
In order to establish a link with respect to a network and perform
data transmission and reception, an STA discovers the network,
performs authentication, establishes association and performs an
authentication process for security. The link setup process may be
referred to as a session initiation process or a session setup
process. In addition, discovery, authentication, association and
security setup of the link setup process may be collectively
referred to as an association process.
An exemplary link setup process will be described with reference to
FIG. 5.
In step S510, the STA may perform a network discovery operation.
The network discovery operation may include a scanning operation of
the STA. That is, the STA discovers the network in order to access
the network. The STA should identify a compatible network before
participating in a wireless network and a process of identifying a
network present in a specific area is referred to as scanning. The
scanning method includes an active scanning method and a passive
scanning method.
In FIG. 5, a network discovery operation including an active
scanning process is shown. In active scanning, the STA which
performs scanning transmits a probe request frame while moving
between channels and waits for a response thereto, in order to
detect which AP is present. A responder transmits a probe response
frame to the STA, which transmitted the probe request frame, as a
response to the probe request frame. The responder may be an STA
which lastly transmitted a beacon frame in a BSS of a scanned
channel. In the BSS, since the AP transmits the beacon frame, the
AP is the responder. In the IBSS, since the STAs in the IBSS
alternately transmit the beacon frame, the responder is not fixed.
For example, the STA which transmits the probe request frame on a
first channel and receives the probe response frame on the first
channel stores BSS related information included in the received
probe response frame, moves to a next channel (e.g., a second
channel) and performs scanning (probe request/response
transmission/reception on the second channel) using the same
method.
Although not shown in FIG. 5, a scanning operation may be performed
using a passive scanning method. In passive scanning, the STA which
performs scanning waits for a beacon frame while moving between
channels. The beacon frame is a management frame in IEEE 802.11 and
is periodically transmitted in order to indicate presence of a
wireless network and to enable the STA, which performs scanning, to
discover and participate in the wireless network. In the BSS, the
AP is responsible for periodically transmitting the beacon frame.
In the IBSS, the STAs alternately transmit the beacon frame. The
STA which performs scanning receives the beacon frame, stores
information about the BSS included in the beacon frame, and records
beacon frame information of each channel while moving to another
channel. The STA which receives the beacon frame may store BSS
related information included in the received beacon frame, move to
a next channel and perform scanning on the next channel using the
same method.
Active scanning has delay and power consumption less than those of
passive scanning.
After the STA has discovered the network, an authentication process
may be performed in step S520. Such an authentication process may
be referred to as a first authentication process to be
distinguished from a security setup operation of step S540.
The authentication process includes a process of, at the STA,
transmitting an authentication request frame to the AP and, at the
AP, transmitting an authentication response frame to the STA in
response thereto. The authentication frame used for authentication
request/response corresponds to a management frame.
The authentication frame may include information about an
authentication algorithm number, an authentication transaction
sequence number, a status code, a challenge text, a robust security
network (RSN), a finite cyclic group, etc. The information may be
examples of information included in the authentication
request/response frame and may be replaced with other information.
The information may further include additional information.
The STA may transmit the authentication request frame to the AP.
The AP may determine whether authentication of the STA is allowed,
based on the information included in the received authentication
request frame. The AP may provide the STA with the authentication
result via the authentication response frame.
After the STA is successfully authenticated, an association process
may be performed in step S530. The association process includes a
process of, at the STA, transmitting an association request frame
to the AP and, at the AP, transmitting an association response
frame to the STA in response thereto.
For example, the association request frame may include information
about various capabilities, beacon listen interval, service set
identifier (SSID), supported rates, RSN, mobility domain, supported
operating classes, traffic indication map (TIM) broadcast request,
interworking service capability, etc.
For example, the association response frame may include information
about various capabilities, status code, association ID (AID),
supported rates, enhanced distributed channel access (EDCA)
parameter set, received channel power indicator (RCPI), received
signal to noise indicator (RSNI), mobility domain, timeout interval
(association comeback time), overlapping BSS scan parameter, TIM
broadcast response, QoS map, etc.
This information is purely exemplary information included in the
association request/response frame and may be replaced with other
information. This information may further include additional
information.
After the STA is successfully authenticated, a security setup
process may be performed in step S540. The security setup process
of step S540 may be referred to as an authentication process
through a robust security network association (RSNA)
request/response. The authentication process of step S520 may be
referred to as the first authentication process and the security
setup process of step S540 may be simply referred to as an
authentication process.
The security setup process of step S540 may include a private key
setup process through 4-way handshaking of an extensible
authentication protocol over LAN (EAPOL) frame. In addition, the
security setup process may be performed according to a security
method which is not defined in the IEEE 802.11 standard.
2.1 Evolution of WLAN
As a technical standard recently established in order to overcome
limitations in communication speed in a WLAN, IEEE 802.11n has been
devised. IEEE 802.11n aims at increasing network speed and
reliability and extending wireless network distance. More
specifically, IEEE 802.11n is based on multiple input and multiple
output (MIMO) technology using multiple antennas in a transmitter
and a receiver in order to support high throughput (HT) with a
maximum data rate of 540 Mbps or more, to minimize transmission
errors, and to optimize data rate.
As WLANs have come into widespread use and applications using the
same have been diversified, recently, there is a need for a new
WLAN system supporting throughput higher than a data rate supported
by IEEE 802.11n. A next-generation WLAN system supporting very high
throughput (VHT) is a next version (e.g., IEEE 802.11ac) of the
IEEE 802.11n WLAN system and is an IEEE 802.11 WLAN system newly
proposed in order to support a data rate of 1 Gbps or more at a MAC
service access point (SAP).
The next-generation WLAN system supports a multi-user MIMO
(MU-MIMO) transmission scheme by which a plurality of STAs
simultaneously accesses a channel in order to efficiently use a
radio channel. According to the MU-MIMO transmission scheme, the AP
may simultaneously transmit packets to one or more MIMO-paired
STAs.
In addition, support of a WLAN system operation in a whitespace is
being discussed. For example, introduction of a WLAN system in a TV
whitespace (WS) such as a frequency band (e.g., 54 to 698 MHz) in
an idle state due to digitalization of analog TVs is being
discussed as the IEEE 802.11af standard. However, this is only
exemplary and the whitespace may be incumbently used by a licensed
user. The licensed user means a user who is allowed to use a
licensed band and may be referred to as a licensed device, a
primary user or an incumbent user.
For example, the AP and/or the STA which operate in the WS should
provide a protection function to the licensed user. For example, if
a licensed user such as a microphone already uses a specific WS
channel which is a frequency band divided on regulation such that a
WS band has a specific bandwidth, the AP and/or the STA cannot use
the frequency band corresponding to the WS channel in order to
protect the licensed user. In addition, the AP and/or the STA must
stop use of the frequency band if the licensed user uses the
frequency band used for transmission and/or reception of a current
frame.
Accordingly, the AP and/or the STA should perform a procedure of
determining whether a specific frequency band in a WS band is
available, that is, whether a licensed user uses the frequency
band. Determining whether a licensed user uses a specific frequency
band is referred to as spectrum sensing. As a spectrum sensing
mechanism, an energy detection method, a signature detection
method, etc. may be used. It may be determined that the licensed
user uses the frequency band if received signal strength is equal
to or greater than a predetermined value or if a DTV preamble is
detected.
In addition, as next-generation communication technology,
machine-to-machine (M2M) communication technology is being
discussed. Even in an IEEE 802.11 WLAN system, a technical standard
supporting M2M communication has been developed as IEEE 802.11ah.
M2M communication means a communication scheme including one or
more machines and may be referred to as machine type communication
(MTC). Here, a machine means an entity which does not require
direct operation or intervention of a person. For example, a device
including a mobile communication module, such as a meter or a
vending machine, may include a user equipment such as a smart phone
which is capable of automatically accessing a network without
operation/intervention of a user to perform communication. M2M
communication includes communication between devices (e.g.,
device-to-device (D2D) communication) and communication between a
device and an application server. Examples of communication between
a device and a server include communication between a vending
machine and a server, communication between a point of sale (POS)
device and a server and communication between an electric meter, a
gas meter or a water meter and a server. An M2M communication based
application may include security, transportation, health care, etc.
If the characteristics of such examples are considered, in general,
M2M communication should support transmission and reception of a
small amount of data at a low rate in an environment in which very
many apparatuses are present.
More specifically, M2M communication should support a larger number
of STAs. In a currently defined WLAN system, it is assumed that a
maximum of 2007 STAs is associated with one AP. However, in M2M
communication, methods supporting the case in which a larger number
of STAs (about 6000) are associated with one AP are being
discussed. In addition, in M2M communication, it is estimated that
there are many applications supporting/requiring a low transfer
rate. In order to appropriately support the low transfer rate, for
example, in a WLAN system, the STA may recognize presence of data
to be transmitted thereto based on a traffic indication map (TIM)
element and methods of reducing a bitmap size of the TIM are being
discussed. In addition, in M2M communication, it is estimated that
there is traffic having a very long transmission/reception
interval. For example, in electricity/gas/water consumption, a very
small amount of data is required to be exchanged at a long period
(e.g., one month). In a WLAN system, although the number of STAs
associated with one AP is increased, methods of efficiently
supporting the case in which the number of STAs, in which a data
frame to be received from the AP is present during one beacon
period, is very small are being discussed.
WLAN technology has rapidly evolved. In addition to the
above-described examples, technology for direct link setup,
improvement of media streaming performance, support of fast and/or
large-scale initial session setup, support of extended bandwidth
and operating frequency, etc. is being developed.
2.2 Medium Access Mechanism
In a WLAN system according to IEEE 802.11, the basic access
mechanism of medium access control (MAC) is a carrier sense
multiple access with collision avoidance (CSMA/CA) mechanism. The
CSMA/CA mechanism is also referred to as a distributed coordination
function (DCF) of IEEE 802.11 MAC and employs a "listen before
talk" access mechanism. According to such an access mechanism, the
AP and/or the STA may perform clear channel assessment (CCA) for
sensing a radio channel or medium during a predetermined time
interval (for example, a DCF inter-frame space (DIFS)) before
starting transmission. If it is determined that the medium is in an
idle state as the sensed result, frame transmission starts via the
medium. If it is determined that the medium is in an occupied
state, the AP and/or the STA may set and wait for a delay period
(e.g., a random backoff period) for medium access without starting
transmission and then attempt to perform frame transmission. Since
several STAs attempt to perform frame transmission after waiting
for different times by applying the random backoff period, it is
possible to minimize collision.
In addition, the IEEE 802.11 MAC protocol provides a hybrid
coordination function (HCF). The HCF is based on the DCF and a
point coordination function (PCF). The PCF refers to a periodic
polling method for enabling all reception AP and/or STAs to receive
data frames using a polling based synchronous access method. In
addition, the HCF has enhanced distributed channel access (EDCA)
and HCF controlled channel access (HCCA). The EDCA uses a
contention access method for providing data frames to a plurality
of users by a provider and the HCCA uses a contention-free channel
access method using a polling mechanism. In addition, the HCF
includes a medium access mechanism for improving quality of service
(QoS) of a WLAN and may transmit QoS data both in a contention
period (CP) and a contention free period (CFP).
FIG. 6 is a diagram illustrating a backoff process.
Operation based on a random backoff period will be described with
reference to FIG. 6. If a medium is changed from an occupied or
busy state to an idle state, several STAs may attempt data (or
frame) transmission. At this time, a method of minimizing
collision, the STAs may select respective random backoff counts,
wait for slot times corresponding to the random backoff counts and
attempt transmission. The random backoff count has a pseudo-random
integer and may be set to one of values of 0 to CW. Here, the CW is
a contention window parameter value. The CW parameter is set to
CWmin as an initial value but may be set to twice CWmin if
transmission fails (e.g., ACK for the transmission frame is not
received). If the CW parameter value becomes CWmax, data
transmission may be attempted while maintaining the CWmax value
until data transmission is successful. If data transmission is
successful, the CW parameter value is reset to CWmin. CW, CWmin and
CWmax values are preferably set to 2n-1 (n=0, 1, 2, . . . ).
If the random backoff process starts, the STA continuously monitors
the medium while the backoff slots are counted down according to
the set backoff count value. If the medium is in the occupied
state, countdown is stopped and, if the medium is in the idle
state, countdown is resumed.
In the example of FIG. 6, if packets to be transmitted to the MAC
of STA3 arrive, STA3 may confirm that the medium is in the idle
state during the DIFS and immediately transmit a frame. Meanwhile,
the remaining STAs monitor that the medium is in the busy state and
wait. During a wait time, data to be transmitted may be generated
in STA1, STA2 and STA5. The STAs may wait for the DIFS if the
medium is in the idle state and then count down the backoff slots
according to the respectively selected random backoff count
values.
In the example of FIG. 6, STA2 selects a smallest backoff count
value and STA1 selects a largest backoff count value. That is, the
residual backoff time of STA5 is less than the residual backoff
time of STA1 when STA2 completes backoff count and starts frame
transmission. STA1 and STA5 stop countdown and wait while STA2
occupies the medium. If occupancy of the medium by STA2 ends and
the medium enters the idle state again, STA1 and STA5 wait for the
DIFS and then resume countdown. That is, after the residual backoff
slots corresponding to the residual backoff time are counted down,
frame transmission may start. Since the residual backoff time of
STA5 is less than of STA1, STA5 starts frame transmission.
If STA2 occupies the medium, data to be transmitted may be
generated in the STA4. At this time, STA4 may wait for the DIFS if
the medium enters the idle state, perform countdown according to a
random backoff count value selected thereby, and start frame
transmission. In the example of FIG. 6, the residual backoff time
of STA5 accidentally matches the random backoff time of STA4. In
this case, collision may occur between STA4 and STA5. If collision
occurs, both STA4 and STA5 do not receive ACK and data transmission
fails. In this case, STA4 and STA5 may double the CW value, select
the respective random backoff count values and then perform
countdown. STA1 may wait while the medium is busy due to
transmission of STA4 and STA5, wait for the DIFS if the medium
enters the idle state, and start frame transmission if the residual
backoff time has elapsed.
2.3 Sensing Operation of STA
As described above, the CSMA/CA mechanism includes not only
physical carrier sensing for directly sensing a medium by an AP
and/or an STA but also virtual carrier sensing. Virtual carrier
sensing solves a problem which may occur in medium access, such as
a hidden node problem. For virtual carrier sensing, MAC of a WLAN
may use a network allocation vector (NAV). The NAV refers to a
value of a time until a medium becomes available, which is
indicated to another AP and/or STA by an AP and/or an STA, which is
currently utilizing the medium or has rights to utilize the medium.
Accordingly, the NAV value corresponds to a period of time when the
medium will be used by the AP and/or the STA for transmitting the
frame, and medium access of the STA which receives the NAV value is
prohibited during that period of time. The NAV may be set according
to the value of the "duration" field of a MAC header of a
frame.
A robust collision detection mechanism for reducing collision has
been introduced, which will be described with reference to FIGS. 7
and 8. Although a transmission range may not be equal to an actual
carrier sensing range, for convenience, assume that the
transmission range may be equal to the actual carrier sensing
range.
FIG. 7 is a diagram illustrating a hidden node and an exposed
node.
FIG. 7(a) shows a hidden node, and, in this case, an STA A and an
STA B are performing communication and an STA C has information to
be transmitted. More specifically, although the STA A transmits
information to the STA B, the STA C may determine that the medium
is in the idle state, when carrier sensing is performed before
transmitting data to the STA B. This is because the STA C may not
sense transmission of the STA A (that is, the medium is busy). In
this case, since the STA B simultaneously receives information of
the STA A and the STA C, collision occurs. At this time, the STA A
may be the hidden node of the STA C.
FIG. 7(b) shows an exposed node and, in this case, the STA B
transmits data to the STA A and the STA C has information to be
transmitted to the STA D. In this case, if the STA C performs
carrier sensing, it may be determined that the medium is busy due
to transmission of the STA B. If the STA C has information to be
transmitted to the STA D, since it is sensed that the medium is
busy, the STA C waits until the medium enters the idle state.
However, since the STA A is actually outside the transmission range
of the STA C, transmission from the STA C and transmission from the
STA B may not collide from the viewpoint of the STA A. Therefore,
the STA C unnecessarily waits until transmission of the STA B is
stopped. At this time, the STA C may be the exposed node of the STA
B.
FIG. 8 is a diagram illustrating request to send (RTS) and clear to
send (CTS).
In the example of FIG. 7, in order to efficiently use a collision
avoidance mechanism, short signaling packet such as RTS and CTS may
be used. RST/CTS between two STAs may be enabled to be overheard by
peripheral STAs such that the peripheral STAs confirm information
transmission between the two STAs. For example, if a transmission
STA transmits an RTS frame to a reception STA, the reception STA
transmits a CTS frame to peripheral UEs to inform the peripheral
UEs that the reception STA receives data.
FIG. 8(a) shows a method of solving a hidden node problem. Assume
that both the STA A and the STA C attempt to transmit data to the
STA B. If the STA A transmits the RTS to the STA B, the STA B
transmits the CTS to the peripheral STA A and C. As a result, the
STA C waits until data transmission of the STA A and the STA B is
finished, thereby avoiding collision.
FIG. 8(b) shows a method of solving an exposed node problem. The
STA C may overhear RTS/CTS transmission between the STA A and the
STA B and determine that collision does not occur even when the STA
C transmits data to another STA (e.g., the STA D). That is, the STA
B transmits the RTS to all peripheral UEs and transmits the CTS
only to the STA A having data to be actually transmitted. Since the
STA C receives the RTS but does not receive the CTS of the STA A,
it can be confirmed that the STA A is outside carrier sensing of
the STA C.
2.4 Power Management
As described above, in a WLAN system, channel sensing should be
performed before an STA performs transmission and reception. When
the channel is always sensed, continuous power consumption of the
STA is caused. Power consumption in a reception state is not
substantially different from power consumption in a transmission
state and continuously maintaining the reception state imposes a
burden on an STA with limited power (that is, operated by a
battery). Accordingly, if a reception standby state is maintained
such that the STA continuously senses the channel, power is
inefficiently consumed without any special advantage in terms of
WLAN throughput. In order to solve such a problem, in a WLAN
system, a power management (PM) mode of the STA is supported.
The PM mode of the STA is divided into an active mode and a power
save (PS) mode. The STA fundamentally operates in an active mode.
The STA which operates in the active mode is maintained in an awake
state. The awake state refers to a state in which normal operation
such as frame transmission and reception or channel scanning is
possible. The STA which operates in the PS mode operates while
switching between a sleep state or an awake state. The STA which
operates in the sleep state operates with minimum power and does
not perform frame transmission and reception or channel
scanning.
Since power consumption is reduced as the sleep state of the STA is
increased, the operation period of the STA is increased. However,
since frame transmission and reception is impossible in the sleep
state, the STA may not unconditionally operate in the sleep state.
If a frame to be transmitted from the STA, which operates in the
sleep state, to the AP is present, the STA may be switched to the
awake state to transmit the frame. If a frame to be transmitted
from the AP to the STA is present, the STA in the sleep state may
not receive the frame and may not confirm that the frame to be
received is present. Accordingly, the STA needs to perform an
operation for switching to the awake state according to a specific
period in order to confirm presence of the frame to be transmitted
thereto (to receive the frame if the frame to be transmitted is
present).
FIG. 9 is a diagram illustrating power management operation.
Referring to FIG. 9, an AP 210 transmits beacon frames to STAs
within a BSS at a predetermined period (S211, S212, S213, S214,
S215 and S216). The beacon frame includes a traffic indication map
(TIM) information element. The TIM information element includes
information indicating that buffered traffic for STAs associated
with the AP 210 is present and the AP 210 will transmit a frame.
The TIM element includes a TIM used to indicate a unicast frame or
a delivery traffic indication map (DTIM) used to indicate a
multicast or broadcast frame.
The AP 210 may transmit the DTIM once whenever the beacon frame is
transmitted three times. An STA1 220 and an STA2 222 operate in the
PS mode. The STA1 220 and the STA2 222 may be switched from the
sleep state to the awake state at a predetermined wakeup interval
to receive a TIM element transmitted by the AP 210. Each STA may
compute a time to switch to the awake state based on a local clock
thereof. In the example of FIG. 9, assume that the clock of the STA
matches the clock of the AP.
For example, the predetermined awake interval may be set such that
the STA1 220 is switched to the awake state every beacon interval
to receive a TIM element. Accordingly, the STA1 220 may be switched
to the awake state (S211) when the AP 210 first transmits the
beacon frame (S211). The STA1 220 may receive the beacon frame and
acquire the TIM element. If the acquired TIM element indicates that
a frame to be transmitted to the STA1 220 is present, the STA1 220
may transmit, to the AP 210, a power save-Poll (PS-Poll) frame for
requesting frame transmission from the AP 210 (S221a). The AP 210
may transmit the frame to the STA1 220 in correspondence with the
PS-Poll frame (S231). The STA1 220 which completes frame reception
is switched to the sleep state.
When the AP 210 secondly transmits the beacon frame, since another
device access the medium and thus the medium is busy, the AP 210
may not transmit the beacon frame at an accurate beacon interval
and may transmit the beacon frame at a delayed time (S212). In this
case, the operation mode of the STA1 220 is switched to the awake
state according to the beacon interval but the delayed beacon frame
is not received. Therefore, the operation mode of the STA1 220 is
switched to the sleep state again (S222).
When the AP 210 thirdly transmits the beacon frame, the beacon
frame may include a TIM element set to a DTIM. Since the medium is
busy, the AP 210 transmits the beacon frame at a delayed time
(S213). The STA1 220 is switched to the awake state according to
the beacon interval and may acquire the DTIM via the beacon frame
transmitted by the AP 210. Assume that the DTIM acquired by the
STA1 220 indicates that a frame to be transmitted to the STA1 220
is not present and a frame for another STA is present. In this
case, the STA1 220 may confirm that a frame transmitted thereby is
not present and may be switched to the sleep state again. The AP
210 transmits the beacon frame and then transmits the frame to the
STA (S232).
The AP 210 fourthly transmits the beacon frame (S214). Since the
STA1 220 cannot acquire information indicating that buffered
traffic therefor is present via reception of the TIM element twice,
the wakeup interval for receiving the TIM element may be
controlled. Alternatively, if signaling information for controlling
the wakeup interval of the STA1 220 is included in the beacon frame
transmitted by the AP 210, the wakeup interval value of the STA1
220 may be controlled. In the present example, the STA1 220 may
change switching of the operation state for receiving the TIM
element every beacon interval to switching of the operation state
every three beacon intervals. Accordingly, since the STA1 220 is
maintained in the sleep state when the AP 210 transmits the fourth
beacon frame (S214) and transmits the fifth beacon frame (S215),
the TIM element cannot be acquired.
When the AP 210 sixthly transmits the beacon frame (S216), the STA1
220 may be switched to the awake state to acquire the TIM element
included in the beacon frame (S224). Since the TIM element is a
DTIM indicating that a broadcast frame is present, the STA1 220 may
not transmit the PS-Poll frame to the AP 210 but may receive a
broadcast frame transmitted by the AP 210 (S234). The wakeup
interval set in the STA2 230 may be set to be greater than that of
the STA1 220. Accordingly, the STA2 230 may be switched to the
awake state to receive the TIM element (S241), when the AP 210
fifthly transmits the beacon frame (S215). The STA2 230 may confirm
that a frame to be transmitted thereto is present via the TIM
element and transmits the PS-Poll frame to the AP 210 (S241a) in
order to request frame transmission. The AP 210 may transmit the
frame to the STA2 230 in correspondence with the PS-Poll frame
(S233).
For PM management shown in FIG. 9, a TIM element includes a TIM
indicating whether a frame to be transmitted to an STA is present
and a DTIM indicating whether a broadcast/multicast frame is
present. The DTIM may be implemented by setting a field of the TIM
element.
FIGS. 10 to 12 are diagrams illustrating operation of a station
(STA) which receives a traffic indication map (TIM).
Referring to FIG. 10, an STA may be switched from a sleep state to
an awake state in order to receive a beacon frame including a TIM
from an AP and interpret the received TIM element to confirm that
buffered traffic to be transmitted thereto is present. The STA may
contend with other STAs for medium access for transmitting a
PS-Poll frame and then transmit the PS-Poll frame in order to
request data frame transmission from the AP. The AP which receives
the PS-Poll frame transmitted by the STA may transmit the frame to
the STA. The STA may receive the data frame and transmit an ACK
frame to the AP. Thereafter, the STA may be switched to the sleep
state again.
As shown in FIG. 10, the AP may receive the PS-Poll frame from the
STA and then operate according to an immediate response method for
transmitting a data frame after a predetermined time (e.g., a short
inter-frame space (SIFS)). If the AP does not prepare a data frame
to be transmitted to the STA during the SIFS after receiving the
PS-Poll frame, the AP may operate according to a deferred response
method, which will be described with reference to FIG. 11.
In the example of FIG. 11, operation for switching the STA from the
sleep state to the awake state, receiving a TIM from the AP,
contending and transmitting a PS-Poll frame to the AP is equal to
that of FIG. 10. If the data frame is not prepared during the SIFS
even when the AP receives the PS-Poll frame, the data frame is not
transmitted but an ACK frame may be transmitted to the STA. If the
data frame is prepared after transmitting the ACK frame, the AP may
contend and transmit the data frame to the STA. The STA may
transmit the ACK frame indicating that the data frame has been
successfully received to the AP and may be switched to the sleep
state.
FIG. 12 shows an example in which the AP transmits the DTIM. The
STAs may be switched from the sleep state to the awake state in
order to receive the beacon frame including the DTIM element from
the AP. The STA may confirm that a multicast/broadcast frame will
be transmitted via the received DTIM. The AP may immediately
transmit data (that is, a multicast/broadcast frame) without
PS-Poll frame transmission and reception after transmitting the
beacon frame including the DTIM. The STAs may receive data in the
awake state after receiving the beacon frame including the DTIM and
may be switched to the sleep state again after completing data
reception.
2.5 TIM Structure
In the PM mode management method based on the TIM (or DTIM)
protocol described with reference to FIGS. 9 to 12, the STAs may
confirm whether a data frame to be transmitted thereto is present
via STA identification included in the TIM element. The STA
identification may be related to an association identifier (AID)
assigned to the STA upon association with the AP.
The AID is used as a unique identifier for each STA within one BSS.
For example, in a current WLAN system, the AID may be one of values
of 1 to 2007. In a currently defined WLAN system, 14 bits are
assigned to the AID in a frame transmitted by the AP and/or the
STA. Although up to 16383 may be assigned as the AID value, 2008 to
16383 may be reserved.
The TIM element according to an existing definition is not
appropriately applied to an M2M application in which a large number
(e.g., more than 2007) of STAs is associated with one AP. If the
existing TIM structure extends without change, the size of the TIM
bitmap is too large to be supported in an existing frame format and
to be suitable for M2M communication considering an application
with a low transfer rate. In addition, in M2M communication, it is
predicted that the number of STAs, in which a reception data frame
is present during one beacon period, is very small. Accordingly, in
M2M communication, since the size of the TIM bitmap is increased
but most bits have a value of 0, there is a need for technology for
efficiently compressing the bitmap.
As an existing bitmap compression technology, a method of omitting
0 which continuously appears at a front part of a bitmap and
defining an offset (or a start point) is provided. However, if the
number of STAs in which a buffered frame is present is small but a
difference between the AID values of the STAs is large, compression
efficiency is bad. For example, if only frames to be transmitted to
only two STAs respectively having AID values of 10 and 2000 are
buffered, the length of the compressed bitmap is 1990 but all bits
other than both ends have a value of 0. If the number of STAs which
may be associated with one AP is small, bitmap compression
inefficiency is not problematic but, if the number of STAs is
increased, bitmap compression inefficiency deteriorates overall
system performance.
As a method of solving this problem, AIDs may be divided into
several groups to more efficiently perform data transmission. A
specific group ID (GID) is assigned to each group. AIDs assigned
based on the group will be described with reference to FIG. 13.
FIG. 13(a) shows an example of AIDs assigned based on a group. In
the example of FIG. 13(a), several bits of a front part of the AID
bitmap may be used to indicate the GID. For example, four DIDs may
be expressed by the first two bits of the AID of the AID bitmap. If
the total length of the AID bitmap is N bits, the first two bits
(B1 and B2) indicate the GID of the AID.
FIG. 13(a) shows another example of AIDs assigned based on a group.
In the example of FIG. 13(b), the GID may be assigned according to
the location of the AID. At this time, the AIDs using the same GID
may be expressed by an offset and a length value. For example, if
GID 1 is expressed by an offset A and a length B, this means that
AIDs of A to A+B-1 on the bitmap have GID 1. For example, in the
example of FIG. 13(b), assume that all AIDs of 1 to N4 are divided
into four groups. In this case, AIDs belonging to GID 1 are 1 to N1
and may be expressed by an offset 1 and a length N1. AIDs belonging
to GID2 may be expressed by an offset N1+1 and a length N2-N1+1,
AIDs belonging to GID 3 may be expressed by an offset N2+1 and a
length N3-N2+1, and AIDs belonging to GID 4 may be expressed by an
offset N3+1 and a length N4-N3+1.
If the AIDs assigned based on the group are introduced, channel
access is allowed at a time interval which is changed according to
the GID to solve lack of TIM elements for a large number of STAs
and to efficiently perform data transmission and reception. For
example, only channel access of STA(s) corresponding to a specific
group may be granted during a specific time interval and channel
access of the remaining STA(s) may be restricted. A predetermined
time interval at which only access of specific STA(s) is granted
may also be referred to as a restricted access window (RAW).
Channel access according to GID will be described with reference to
FIG. 13(c). FIG. 13(c) shows a channel access mechanism according
to a beacon interval if the AIDs are divided into three groups. At
a first beacon interval (or a first RAW), channel access of STAs
belonging to GID 1 is granted but channel access of STAs belonging
to other GIDs is not granted. For such implementation, the first
beacon includes a TIM element for AIDs corresponding to GID 1. A
second beacon frame includes a TIM element for AIDs corresponding
to GID 2 and thus only channel access of the STAs corresponding to
the AIDs belonging to GID 2 is granted during the second beacon
interval (or the second RAW). A third beacon frame includes a TIM
element for AIDs corresponding to GID 3 and thus only channel
access of the STAs corresponding to the AIDs belonging to GID 3 is
granted during the third beacon interval (or the third RAW). A
fourth beacon frame includes a TIM element for AIDs corresponding
to GID 1 and thus only channel access of the STAs corresponding to
the AIDs belonging to GID 1 is granted during the fourth beacon
interval (or the fourth RAW). Only channel access of the STAs
corresponding to a specific group indicated by the TIM included in
the beacon frame may be granted even in fifth and subsequent beacon
intervals (or fifth and subsequent RAWs).
Although the order of GIDs allowed according to the beacon interval
is cyclic or periodic in FIG. 13(c), the present invention is not
limited thereto. That is, by including only AID(s) belonging to
specific GID(s) in the TIM elements, only channel access of STA(s)
corresponding to the specific AID(s) may be granted during a
specific time interval (e.g., a specific RAW) and channel access of
the remaining STA(s) may not be granted.
The above-described group based AID assignment method may also be
referred to as a hierarchical structure of a TIM. That is, an
entire AID space may be divided into a plurality of blocks and only
channel access of STA(s) corresponding to a specific block having a
non-zero value (that is, STAs of a specific group) may be granted.
A TIM having a large size is divided into small blocks/groups such
that the STA easily maintains TIM information and easily manages
blocks/groups according to class, QoS or usage of the STA. Although
a 2-level layer is shown in the example of FIG. 13, a TIM of a
hierarchical structure having two or more levels may be
constructed. For example, the entire AID space may be divided into
a plurality of page groups, each page group may be divided into a
plurality of blocks, and each block may be divided into a plurality
of sub-blocks. In this case, as an extension of the example of FIG.
13(a), the first N1 bits of the AID bitmap indicate a paging ID
(that is, a PID), the next N2 bits indicate a block ID, the next N3
bits indicate a sub-block ID, and the remaining bits indicate the
STA bit location in the sub-block.
In the following examples of the present invention, various methods
of dividing and managing STAs (or AIDs assigned to the STAs) on a
predetermined hierarchical group basis are applied and the group
based AID assignment method is not limited to the above
examples.
2.6 Improved Channel Access Method
If AIDs are assigned/managed based on a group, STAs belonging to a
specific group may use a channel only at a "group channel access
interval (or RAW)" assigned to the group. If an STA supports an M2M
application, traffic for the STA may have a property which may be
generated at a long period (e.g., several tens of minutes or
several hours). Since such an STA does not need to be in the awake
state frequently, the STA may be in the sleep mode for g a long
period of time and be occasionally switched to the awake state
(that is, the awake interval of the STA may be set to be long). An
STA having a long wakeup interval may be referred to as an STA
which operates in a "long-sleeper" or "long-sleep" mode. The case
in which the wakeup interval is set to be long is not limited to
M2M communication and the wakeup interval may be set to be long
according to the state of the STA or surroundings of the STA even
in normal WLAN operation.
If the wakeup interval is set, the STA may determine whether a
local clock thereof exceeds the wakeup interval. However, since the
local clock of the STA generally uses a cheap oscillator, an error
probability is high. In addition, if the STA operates in long-sleep
mode, the error may be increased with time. Accordingly, time
synchronization of the STA which occasionally wakes up may not
match time synchronization of the AP. For example, although the STA
computes when the STA may receive the beacon frame to be switched
to the awake state, the STA may not actually receive the beacon
frame from the AP at that timing. That is, due to clock drift, the
STA may miss the beacon frame and such a problem may frequently
occur if the STA operates in the long sleep mode.
FIGS. 14 to 16 are diagrams showing examples of operation of an STA
if a group channel access interval is set.
In the example of FIG. 14, STA3 may belong to group 3 (that is,
GID=3), wake up at a channel access interval assigned to group 1
and perform PS-Poll for requesting frame transmission from the AP.
The AP which receives PS-Poll from the STA transmits an ACK frame
to STA3. If buffered data to be transmitted to STA3 is present, the
AP may provide information indicating that data to be transmitted
is present via the ACK frame. For example, the value of a "More
Data" field (or an MD field) having a size of 1 bit included in the
ACK frame may be set to 1 (that is, MD=1) to indicate the above
information.
Since a time when STA3 transmits PS-Poll belongs to the channel
access interval for group 1, even if data to be transmitted to STA3
is present, the AP does not immediately transmit data after
transmitting the ACK frame but transmits data to STA3 at a channel
access interval (GID 3 channel access of FIG. 14) assigned to group
3 to which STA3 belongs.
Since STA3 receives the ACK frame set to MD=1 from the AP, STA3
continuously waits for transmission of data from the AP. That is,
in the example of FIG. 14, since STA3 cannot receive the beacon
frame immediately after waking up, STA3 transmits PS-Poll to the AP
on the assumption that a time when STA3 wakes up corresponds to the
channel access interval assigned to the group, to which STA3
belongs, according to computation based on the local clock thereof
and data to be transmitted thereto is present. Alternatively, since
STA3 operates in the long-sleep mode, on the assumption that time
synchronization is not performed, if the data to be transmitted
thereto is present, STA3 may transmit PS-Poll to the AP in order to
receive the data. Since the ACK frame received by STA3 from the AP
indicates that data to be transmitted to STA3 is present, STA3
continuously waits for data reception under the assumption of the
interval in which channel access thereof is granted. STA3
unnecessarily consumes power even when data reception is not
allowed, until time synchronization is appropriately performed from
information included in a next beacon frame.
In particular, if STA3 operates in the long-sleep mode, the beacon
frame may frequently not be received, CCA may be performed even at
the channel access interval, to which STA2 does not belong, thereby
causing unnecessary power consumption.
Next, in the example of FIG. 15, the beacon frame is missed when
the STA having GID 1 (that is, belonging to group 1) wakes up. That
is, the STA which does not receive the beacon frame including the
GID (or PID) assigned thereto is continuously in the awake state
until the beacon frame including the GID (or PID) thereof is
received. That is, although the STA wakes up at channel access
interval assigned thereto, the STA cannot confirm whether the GID
(or PID) thereof is included in the TIM transmitted via the beacon
frame and thus cannot confirm whether the timing corresponds to the
channel access interval assigned to the group thereof.
In the example of FIG. 15, the STA which is switched from the sleep
state to the awake state is continuously in the awake state until
the fourth beacon frame including the GID (that is, GID 1) thereof
is received after the first beacon frame has been missed, thereby
causing unnecessary power consumption. As a result, after
unnecessary power consumption, the STA may receive the beacon frame
including GID 1 and then may perform RTS transmission, CTS
reception, data frame transmission and ACK reception.
FIG. 16 shows the case in which an STA wakes up at a channel access
interval for another group. For example, the STA having GID 3 may
wake up at the channel access interval for GID 1. That is, the STA
having GID 3 unnecessarily consumes power until the beacon frame
having the GID thereof is received after waking up. If a TIM
indicating GID 3 is received via a third beacon frame, the STA may
recognize the channel access interval for the group thereof and
perform data transmission and ACK reception after CCA through RTS
and CTS.
3. Proposed Pilot Sequence Transmission and Reception Methods
As interest in future Wi-Fi and demand for improvement of yield and
QoE (quality of experience) after 802.11ac increase, it is
necessary to define a new frame format for future WLAN systems. The
most important part in a new frame format is a preamble part
because design of a preamble used for synchronization, channel
tracking, channel estimation, adaptive gain control (AGC) and the
like may directly affect system performance.
In the future Wi-Fi system in which a large number of APs and STAs
simultaneously access and attempt data transmission and reception,
system performance may be limited when legacy preamble design is
employed. That is, if each preamble block (e.g., a short training
field (STF) in charge of AGC, CFO estimation/compensation, timing
control and the like or a long training field (LTF) in charge of
channel estimation/compensation, residual CFO compensation and the
like) executes only the function thereof defined in the legacy
preamble structure, frame length increases, causing overhead.
Accordingly, if a specific preamble block can support various
functions in addition to the function designated therefor, an
efficient frame structure can be designed.
Furthermore, since the future Wi-Fi system considers data
transmission in outdoor environments as well as indoor
environments, the preamble structure may need to be designed
differently depending on environments. Although design of a unified
preamble format independent of environment variation can aid in
system implementation and operation, of course, it is desirable
that preamble design be adapted to system environment.
Preamble design for efficiently supporting various functions is
described hereinafter. For convenience, a new WLAN system is
referred to as an HE (High Efficiency) system and a frame and a
PPDU (PLCP (Physical Layer Convergence Procedure) Protocol Data
Unit) of the HE system are respectively referred to as an HE frame
and an HE PPDU. However, it is obvious to those skilled in the art
that the proposed preamble is applicable to other WLAN systems and
cellular systems in addition to the HE system.
The following table 1 shows OFDM numerology which is a premise of a
pilot sequence transmission method described below. Table 1 shows
an example of new OFDM numerology proposed in the HE system and
numerals and items shown in Table 1 are merely examples and other
values may be applied. Table 1 is based on the assumption that FFT
having a size four times the legacy one is applied to a given BW
and 3 DCs are used per BW.
TABLE-US-00001 TABLE 1 Parameter CBW20 CBW40 CBW80 CBW80 + 80
CBW160 Description N.sub.FFT 256 512 1024 1024 2048 FFT size
N.sub.SD 238 492 1002 1002 2004 Number of complex data numbers per
frequency segment N.sub.SP 4 6 8 8 16 Number of pilot values per
frequency segment N.sub.ST 242 498 1010 1010 2020 Total number of
subcarriers per frequency segment. See NOTE. N.sub.SR 122 250 506
506 1018 Highest data subcarrier index per frequency segment
N.sub.Seg 1 1 1 2 1 Number of frequency segments .DELTA..sub.F
312.5 kHz Subcarrier frequency Spacing for non-HE portion
.DELTA..sub.F HE 78.125 kHz Subcarrier frequency Spacing for HE
portion T.sub.DFT 3.2 .mu.s IDFT/DFT period for non-HE portion
T.sub.DFT_HE 12.8 .mu.s IDFT/DFT period for HE portion T.sub.GI 0.8
.mu.s = T.sub.DFT/4 Guard interval duration for non- HE portion
T.sub.GI_HE 3.2 .mu.s = T.sub.DFT_HE/4 Guard interval duration for
HE portion T.sub.GI2 1.6 .mu.s Double guard interval for non-HE
portion T.sub.GIS HE 0.8 .mu.s = T.sub.DFT_HE/16 Short guard
interval [Alternative: 0.4 .mu.s (1/32 CP)] Duration (used only for
HE data) T.sub.SYML 4 .mu.s = T.sub.DFT + T.sub.GI Long GI symbol
interval for non-HE portion T.sub.SYML_HE 16 .mu.s = T.sub.DFT_HE +
T.sub.GI_HE Long GI symbol interval for HE portion T.sub.SYMS_HE
13.6 .mu.s = T.sub.DFT_HE + T.sub.GIS_HE Short GI symbol
[Alternative: 13.2 .mu.s (with 1/32 CP)] interval (used only for HE
data) T.sub.SYM T.sub.SYML or T.sub.SYMS depending on the GI used
Symbol interval for non-HE portion T.sub.SYM HE T.sub.SYML_HE or
T.sub.SYMS_HE depending on the GI used Symbol interval for HE
portion T.sub.L-STF 8 .mu.s = 10 * T.sub.DFT/4 Non-HE Short
Training field duration T.sub.L-LTF 8 .mu.s = 2 .times. T.sub.DFT +
T.sub.GI2 Non-HE Long Training field duration T.sub.L-SIG 4 .mu.s =
T.sub.SYML Non-HE SIGNAL field duration T.sub.HE-SIGA 12.8 .mu.s =
2(T.sub.SYML + 3T.sub.GI) in HE- HE Signal A field PPDU format-1 or
T.sub.SYML_HE in HE- duration PPDU format-2 and HE-PPDU format-3
T.sub.HE-STF T.sub.SYML_HE HE Short Training field duration
T.sub.HE-LTF T.sub.SYML_HE Duration of each HE LTF symbol
T.sub.HE-SIGB T.sub.SYML_HE HE Signal B field duration
N.sub.service 16 Number of bits in the SERVICE field N.sub.tail 6
Number of tail bits per BCC encoder NOTE N.sub.ST = N.sub.SD +
N.sub.SP
FIG. 17 is a diagram illustrating frame structures related to an
embodiment of the present invention. As illustrated in FIGS. 17(a),
17(b) and 17(c), various frame structures can be configured, and a
proposed pilot sequence transmission method is related to an HE-STF
(High Efficiency Short Training Field) in a preamble in a frame
structure.
FIG. 18 is a diagram illustrating a pilot sequence related to an
embodiment of the present invention. The HE-STF illustrated in FIG.
17 is a part of a preamble and carries pilot signals for channel
estimation, CFO (Carrier Frequency Offset) estimation, symbol
timing estimation and the like. Sequentially transmitted pilot
signals are called a pilot sequence. FIG. 18 illustrates general
design of a pilot sequence in the HE-STF. In FIG. 18, the upper
graph shows a pilot sequence in the frequency domain and the lower
graph shows a pilot sequence in the time domain.
The pilot sequence in the frequency domain is defined by a guard
interval, a pilot distance and a pilot signal magnitude. In the
example shown in FIG. 18, small arrows in the pilot sequence in the
frequency domain indicate pilot signals having a magnitude of 0 and
a distance between neighboring pilot signals is 2 (i.e., pilot
distance=2). When the guard interval is a multiple of the pilot
distance (a multiple of 2), if pilot signals in the frequency
domain are transformed into the time domain, pilot signals in the
time domain have a repeated pattern as shown in the lower part of
FIG. 18. When signals in a first period are referred to as first
signals and signals in a second period are referred to as second
signals, the first signals and the second signals have the same
pattern.
If the pilot distance of the pilot signals in the frequency domain
is 4 and the guard interval is a multiple of 4, the pilot signals
in the time domain are defined as 4 repeated patterns and
first/second/third/fourth signals have the same pattern. Such
definition of signals in a repeated pattern in the time domain is
meaningful because it enables accurate symbol timing and CFO
estimation without channel information.
FIG. 19 is a diagram illustrating cyclic shifting of a pilot
sequence. FIG. 19 illustrates first signals in the time domain,
shown in FIG. 18, and signals shifting from the first signals. A
plurality of pilot sequences generated through shifting of a
specific pilot sequence is called a sequence set, and a transmitter
selects a pilot sequence from a sequence set and transmits the
selected pilot sequence to a receiver. Shifting of a pilot signal
in the time domain can be understood as a process of changing the
phase of the pilot signal, whereas shifting of a pilot signal in
the frequency domain can be understood as cyclic shifting.
In FIG. 19, a transmitter selects a pilot sequence (or simply a
sequence) from a sequence set composed of 4 pilot sequences
(shifting=0, 1, 2 and 3) generated through shifting and transmits
the selected pilot sequence to a receiver. The receiver can
identify the received pilot sequence from among the 4 pilot
sequences. This procedure can be performed through a process of
calculating correlation with respect to received signal by the
receiver. The receiver can detect the received pilot sequence by
comparing results of correlation between previously stored pilot
sequences and received signals.
As the receiver can discriminate the 4 pilot sequences, the
transmitter can transmit 2 bits of additional information to the
receiver. That is, upon determining that a case of transmitting a
pilot sequence of shifting=0 and a case of transmitting a pilot
sequence of shifting=1 indicate different pieces of information
between the transmitter and the receiver, the receiver can acquire
the additional information corresponding to 2 bits depending on
which pilot sequence in the sequence set is received. This
additional information is called a "signature".
In FIG. 19, four different pilot sequences can be transmitted to
the receiver and the receiver can discriminate the four pilot
sequences. Transmitted pilot sequences in the sequence set
respectively correspond to four different pieces of information,
and thus the receiver can acquire additional information (i.e., a
signature) corresponding to 2 bits. Specifically, the receiver can
recognize that 2 bits information corresponding to "00" is received
when the pilot sequence of shifting=0 is received, recognize that 2
bits information corresponding to "01" is received when the pilot
sequence of shifting=1 is received, recognize that 2 bits
information corresponding to "10" is received when the pilot
sequence of shifting=2 is received and recognize that 2 bits of
information corresponding to "11" is received when the pilot
sequence of shifting=3 is received.
FIG. 20 is a diagram illustrating a receiver structure for
identification of a pilot sequence and FIG. 21 is a diagram
illustrating signals of a sequence set received by the receiver. A
description will be given of FIGS. 20 and 21.
A receiver structure for identifying a received pilot sequence may
be implemented as shown in FIG. 20. In FIG. 20, N is the length of
a first signal, r is a received vector in the time domain,
t.sup.(i) is the vector of an i-th shifting pilot sequence and
y.sub.i=|r.sup.it.sup.(i)|.
An environment having no AWGN (Additive White Gaussian Noise)
channel and noise is assumed. When the transmitter transmits the
first sequence from among the pilot sequences shown in FIG. 19, the
receiver calculates {y0, y1, y2, y3} as shown in FIG. 21. Then, the
receiver can identify the sequence transmitted by the transmitter
from the sequence set by detecting yi having the highest magnitude
from among {y0, y1, y2, y3}.
FIG. 22 is a diagram illustrating a sequence set composed of pilot
sequences generated at a predetermined interval.
Meanwhile, the aforementioned communication method of the
transmitter and the receiver has a disadvantage of performance
deterioration in multipath environments. To overcome this
disadvantage, pilot sequences of a sequence set are generated at a
predetermined interval and used, as illustrated in FIG. 22.
Compared to the pilot sequences of FIG. 19, the pilot sequences
shown in FIG. 22 are generated at a shifting interval of 4. The
shifting interval is determined by a channel effective delay period
L. When the transmitter transmits a sequence with shifting-0 in a
noise-free environment, the receiver having the structure shown in
FIG. 20 can calculate a received signal yi as illustrated in FIG.
23.
FIG. 23 is a diagram illustrating received signals of the sequence
set illustrated in FIG. 22. In FIG. 23, the size W of a zero
correlation zone (ZCZ) is determined as a maximum shifting value L
of the sequence (W=L). The receiver selects the highest signal yi
in each ZCZ and compares highest signals yi selected in respective
ZCZs to select a ZCZ having the highest value. Referring to FIG.
23, the first ZCZ has {y0, y1, y2, y3} higher than other ZCZs,
which is caused by delay spread of transmitted signals due to
reception through multiple paths. The receiver can identify a
sequence transmitted by the transmitter without error by setting a
ZCZ larger than the effective channel delay period.
When the sequence set shown in FIG. 19 instead of the sequence set
shown in FIG. 22 is used, the size of the ZCZ is 1 (W=1). In this
case, it is necessary to identify a sequence by comparing all
results {y.sub.0, y.sub.1, . . . , y.sub.N-1} of processing of
received signals because y.sub.0, y.sub.1, . . . , y.sub.N-1 are
representative values of respective ZCZs. If the transmitter
transmits a sequence with shifting=0, one of {y1, y2, y3} may have
a value greater than y.sub.0, abruptly decreasing the
identification performance of the receiver.
As the sequence interval decreases, the number of signatures that
can be generated increases. For example, when the sequence interval
is set to 1 in FIG. 22, a total of 16 signatures can be defined.
That is, as the interval of pilot sequences forming a sequence set
decreases, the number of generated signatures can increase to
transmit more information. In this case, however, tradeoff of
deterioration of the identification performance of the receiver is
generated.
FIG. 24 is a diagram illustrating a timing offset. FIG. 24 shows a
process of receiving one OFDM symbol through two paths. It can be
confirmed that a signal received through the second path is delayed
from a signal transmitted through the first path. In general, a
receiver estimates the start point of an OFDM symbol as a point
between the guard interval (GI) start point of the first path and
the GI end point of the second path. With respect to the estimated
start point (ESP), a timing offset NTO is represented by the
following mathematical expression 1. That is, the timing offset is
represented as a difference between the ESP and the start point
(SP) of the first symbol. N.sub.TO=ESP-1st SP [Expression 1]
If the receiver determines the ESP within an
inter-symbol-interference-free (ISI) period, ISI can be avoided.
That is, orthogonality between subcarriers can be maintained in an
OFDM symbol.
FIG. 25 is a diagram illustrating received signals considering a
timing offset. The timing offset described above affects the
signature identification performance of the receiver. For example,
when the timing offset N.sub.TO is 4, FIG. 23 is modified into FIG.
25.
Distinguished from FIG. 23, all values yi are shifted by 4 in FIG.
25. If the receiver knows the timing offset, the receiver can
correctly identify the received sequence by shifting a window by
the timing offset. If the receiver does not know the timing offset,
the receiver misrecognizes the shifting value of the received
sequence and thus determines that an incorrect sequence has been
received.
FIG. 26 is a diagram illustrating a procedure of controlling the
size of the ZCZ in consideration of a timing offset. FIG. 26
illustrates a method for solving the aforementioned problem.
In FIG. 26, a maximum timing offset that can be generated is
defined as N.sub.TO and a minimum shifting value between sequences
forming a sequence set in a process of generating the sequence set
is defined as L+N.sub.TO. In addition, the receiver defines the
size of the ZCZ as L+N.sub.TO corresponding to the sum of the
effective channel delay period and TO. Distinguished from FIG. 25,
an error caused by the timing offset is not generated because the
window is sufficiently large in FIG. 26. On the other hands, the
large window causes tradeoff that the number of signatures that can
be identified between the transmitter and the receiver is reduced
from 4 to 2.
FIG. 27 is a diagram illustrating a pilot sequence using a CAZAC
(Constant Amplitude Zero Auto Correlation) sequence.
FIG. 27 illustrates an embodiment in which a Chu sequence having a
length of N.sub.s is used as pilot signals in the frequency domain.
In FIG. 27, it is assumed that OFMD symbol length N.sub.o satisfies
N.sub.o=.alpha.N.sub.s (.alpha. is a positive number greater than
0). Here, first signals and second signals have properties of the
CAZAC sequence.
All pilot sequences of the time domain have a constant amplitude.
In addition, the sequences have zero auto-correlation except
themselves. That is, first signals and second signals have CAZAC
properties.
When the CAZAC sequence is used in the time domain as described
above, various advantages can be obtained. All pilot sequences have
a magnitude of 1 and thus PAPR (Peak to Average Power Ratio)
becomes 1. The PAPR is defined as the ratio of peak power to
average power and has a minimum value of 1. The peak power means
highest power from among powers of time domain elements and the
average power means average power of the time domain elements.
When a transmitter amplifies OFDM symbols, a maximum number of OFDM
symbols that can be amplified is determined by an element having a
maximum magnitude from among the time domain elements. This is
because all time elements of OFDM symbols should not exceed peak
power when the OFDM symbols are amplified by an amplifier.
Accordingly, when all symbols have a magnitude of 1, sequences can
be amplified to peak power of the amplifier and transmitted. That
is, OFDM symbols can be amplified to peak power of the amplifier
and transmitted, increasing SNR.
When a sequence satisfying zero auto-correlation (ZAC) is used in
the time domain, y.sub.1, y.sub.2 and y.sub.3 become 0 in FIG. 23.
This property improves identification performance of the receiver.
If a sequence having no ZAC property is used, all y.sub.1, y.sub.2
and y.sub.3 are non-zero, decreasing identification performance of
the receiver. In other words, identification performance of the
receiver is reduced when sequences have a correlation
therebetween.
A description will be given of a proposed pilot sequence
transmission method with reference to FIGS. 28 to 32.
A transmitter generates a baseline sequence and then shifts the
baseline sequence in the time domain to generate a plurality of
pilot sequences. The generated pilot sequences form a sequence set
and are mapped to different pieces of additional information (i.e.,
a signature) as described above.
While the CAZAC sequence has been described above, a guard interval
(or a guard band) is present before pilot signals in the frequency
domain, similarly to the structure shown in FIG. 18, in an actually
implemented system. Accordingly, a pilot sequence does not have the
CAZAC properties as in FIG. 27, resulting in
N.sub.o.noteq..alpha.N.sub.s. For example, FIG. 28 illustrates the
magnitude and auto-correlation of first signals when N.sub.o=256
and N.sub.s=122. Referring to FIG. 28, samples have different
magnitudes (non-constant amplitude) and non-zero auto-correlation
at non-zero shifting values can be confirmed. A sequence having
such properties is called a non-CAZAC sequence hereinafter.
In the case of non-CAZAC sequence, sequences have a non-zero
correlation therebetween and {y.sub.1, y.sub.2, y.sub.3}
illustrated in FIG. 21 have values greater than 0. FIG. 29 shows
that all elements yi have non-zero values when the non-CAZAC
sequence is used. This means that non-zero values are generated in
zones other than a ZCZ with respect to a pilot sequence transmitted
by the transmitter, and thus performance of the receiver is
deteriorated compared to cases using the CAZAC sequence. A degree
of performance deterioration is determined by a largest value from
among values generated in ZCZs to which a received pilot sequence
does not belong. For example, y.sub.13 determines identification
performance of the receiver in FIG. 29.
In view of this, the transmitter can configure a sequence set in
consideration of a correlation between sequences in a procedure of
designing the sequence set based on a non-CAZAC sequence.
Specifically, in a procedure of shifting the baseline sequence to
generate pilot sequences, the transmitter according to an
embodiment determines a shifting value such that a correlation
between pilot sequences is minimized.
FIG. 30 illustrates variations in a correlation between sequences
depending on shifting value differences. For example, when a
correlation corresponding to a shifting value of 0 is defined as 1,
it can be seen from FIG. 30 that correlations are -13 dB and -30-dB
when the shifting value is 1 and 21, respectively. Accordingly, the
transmitter can generate a sequence set in consideration of
correlation variations depending on shifting value differences.
This will be described below in detail.
In the following, t.sup.(0) denotes a baseline sequence and
t.sup.(i) denotes a pilot sequence shifted from the baseline
sequence by i. Two sequence sets as represented by the following
mathematical expression 2 can be considered with reference to FIG.
30. Set A: {t.sup.(0),t.sup.(1),t.sup.(2),t.sup.(3), . . .
,t.sup.(127)} Set B: {t.sup.(11),t.sup.(21),t.sup.(42),t.sup.(63)}
[Expression 2]
In the case of sequence set A, a sequence distance (i.e., shifting
value) is 1 and a largest correlation difference is -13 dB. In the
case of sequence set B, a sequence distance is 21 and a largest
correlation difference is -29 dB. Since properties of sequences
become closer to CAZAC properties to improve receiver performance
as a correlation between the sequences decreases, sequence set B
provides better receiver identification performance than sequence
set A. However, when sequence set A is used, a larger number of
signatures than sequence set B can be defined and transmitted to
the receiver.
As described above, the transmitter can determine a shifting value
such that a correlation between pilot sequences is minimized to
configure a sequence set. According to another embodiment, a
minimum shifting value can be determined in consideration of a
channel effective delay period L and a timing offset N.sub.TO in
the procedure of generating a sequence set.
The channel effective delay period L can be defined to be shorter
than delay spread length of an actual channel. The influence of
delay spread on receiver performance has been described with
reference to FIG. 23 and the influence of a timing offset on
receiver performance has been described with reference to FIG. 25.
In addition, performance deterioration due to a correlation between
sequences when the non-CAZAC sequence is used has been described
with reference to FIG. 29.
Accordingly, in determination of a shifting value of pilot
sequences, a minimum shifting value can be determined on the basis
of the channel effective delay period L and the timing offset
N.sub.TO and then a shifting value capable of minimizing a
correlation between sequences can be finally determined on the
basis of the determined minimum shifting value.
For example, the transmitter determines a minimum shifting value as
L+N.sub.TO as illustrated in FIG. 26. Then, the transmitter
determines a shifting value capable of minimizing a correlation
between pilot sequences using auto-correlation values described
with reference to FIG. 30. A sequence set generated through this
procedure secures receiver performance robust against delay spread
and timing offset. This causes tradeoff that a maximum number of
signatures that can be defined by the generated sequence set is
limited to mathematical expression 3.
.function..times..times. ##EQU00001##
If the receiver can estimate the timing offset, the receiver can
shift a window by the timing offset to correctly identify a
received pilot sequence, as illustrated in FIG. 25. In this case,
the transmitter may not consider the timing offset in the procedure
of generating the sequence set and may determine the minimum
shifting value as L without considering the timing offset. In this
embodiment, a maximum number of defined signatures is represented
by mathematical expression 4.
.times..times. ##EQU00002##
A description will be given of an embodiment using a quantized Chu
sequence with reference to FIG. 31.
The procedure of generating first signals and second signals using
a Chu sequence as a pilot sequence has been described with
reference to FIG. 27. Values of samples of the Chu sequence are
defined through subdivided phase differences, and hardware of the
transmitter needs to discriminate and control such phases in order
to support the Chu sequence. If not, an error vector magnitude
(EVM) may be generated. The EVM refers to a problem that a
transmitter transmits signals out of points on a constellation
which are originally intended to be transmitted due to imperfect
hardware. For example, although the transmitter intends to transmit
QPSK signals denoted by a square in FIG. 31, the transmitter
transmits a signal denoted by a triangle when EVM is generated.
To solve this problem, the transmitter can quantize a Chu sequence
and then use the quantize Chu sequence as pilot signals. Then, the
transmitter generates a baseline sequence in the time domain
through IFFT (Inverse Fast Fourier Transform). In this manner, the
transmitter can generate a baseline sequence and a sequence set in
consideration of accuracy of phase control supportable by
hardware.
A sequence set based on the quantized Chu sequence has an
increasing PAPR compared to a sequence set generated based on the
Chu sequence. The following table 2 shows root indices and PAPRs of
a pilot sequence according to constellations when N.sub.o=256 and
N.sub.s=122.
TABLE-US-00002 TABLE 2 Root index 1 15 BPSK 4.2039 4.5864 QPSK
2.5323 3.5769 8-PSK 1.9833 2.9272 16-PSK 1.8554 3.545 32-PSK 1.8295
2.8135 Chu 1.7826 2.9513
As can be seen from Table 2, PAPR decreases as constellation
increases when the same root index is used. In Table 2, "Chu"
represents a non-quantized case and has a PAPR gain of 0.46 dB
compared to the 8-PSK (Phase Shift Keying) case. Accordingly, a
sufficiently low PAPR can be provided through the quantized Chu
sequence even when 8-PSK is used.
In FIG. 31, points denoted by a circle represent the phase of the
Chu sequence before quantization and points denoted by a square
represent the phase of QPSK (Quadrature Phase Shift Keying).
Quantization of the Chu sequence through QPSK refers to a procedure
of approximating the points denoted by a circle to closest square
points.
As described above, the transmitter can generate a sequence set
such that a correlation between pilot sequences is minimized when a
non-CAZAC sequence is used. Accordingly, the transmitter can secure
performance of the receiver while transmitting a signature.
FIG. 32 is a flowchart illustrating operation processes of a
transmitter and a receiver according to a proposed embodiment. FIG.
32 illustrates a time series flow based on the aforementioned
embodiments and the above description can be equally or similarly
applied to the flow even if the flow is not described in
detail.
The transmitter generates a sequence set such that a correlation
between pilot sequences is minimized (S3210). This process can be
performed through a procedure of controlling a shifting value
during shifting of a baseline sequence. For example, the
transmitter can determine a minimum shifting value in consideration
of a channel effective delay period or a timing offset. The
transmitter can finally determine a shifting value that minimizes a
correlation between pilot sequences in consideration of a
correlation between pilot sequences depending on a shifting value.
Alternatively, the transmitter may configure a sequence set using a
quantized Chu sequence.
Subsequently, the transmitter transmits a pilot sequence selected
from the generated sequence set (S3220). Pilot sequences included
in the sequence set are mapped to additional information (a
signature) represented by different bits. Accordingly, the receiver
receives the pilot sequence (S3230) and obtains additional
information transmitted by the transmitter by identifying the
received pilot sequence (S3240). That is, the receiver can
recognize the additional information intended to be transmitted by
the transmitter by identifying the shifting value of the received
pilot sequence.
4. Apparatus Configuration
FIG. 33 is a block diagram showing the configuration of a reception
module and a transmission module according to one embodiment of the
present invention. In FIG. 33, a reception module 100 and the
transmission module 150 may include radio frequency (RF) units 110
and 160, processors 120 and 170 and memories 130 and 180,
respectively. Although a 1:1 communication environment between the
reception module 100 and the transmission module 150 is shown in
FIG. 33, a communication environment may be established between a
plurality of reception module and the transmission module. In
addition, the transmission module 150 shown in FIG. 33 is
applicable to a macro cell base station and a small cell base
station.
The RF units 110 and 160 may include transmitters 112 and 162 and
receivers 114 and 164, respectively. The transmitter 112 and the
receiver 114 of the reception module 100 are configured to transmit
and receive signals to and from the transmission module 150 and
other reception modules and the processor 120 is functionally
connected to the transmitter 112 and the receiver 114 to control a
process of, at the transmitter 112 and the receiver 114,
transmitting and receiving signals to and from other apparatuses.
The processor 120 processes a signal to be transmitted, sends the
processed signal to the transmitter 112 and processes a signal
received by the receiver 114.
If necessary, the processor 120 may store information included in
an exchanged message in the memory 130. By this structure, the
reception module 100 may perform the methods of the various
embodiments of the present invention.
The transmitter 162 and the receiver 164 of the transmission module
150 are configured to transmit and receive signals to and from
another transmission module and reception modules and the processor
170 are functionally connected to the transmitter 162 and the
receiver 164 to control a process of, at the transmitter 162 and
the receiver 164, transmitting and receiving signals to and from
other apparatuses. The processor 170 processes a signal to be
transmitted, sends the processed signal to the transmitter 162 and
processes a signal received by the receiver 164. If necessary, the
processor 170 may store information included in an exchanged
message in the memory 180. By this structure, the transmission
module 150 may perform the methods of the various embodiments of
the present invention.
The processors 120 and 170 of the reception module 100 and the
transmission module 150 instruct (for example, control, adjust, or
manage) the operations of the reception module 100 and the
transmission module 150, respectively. The processors 120 and 170
may be connected to the memories 130 and 180 for storing program
code and data, respectively. The memories 130 and 180 are
respectively connected to the processors 120 and 170 so as to store
operating systems, applications and general files.
The processors 120 and 170 of the present invention may be called
controllers, microcontrollers, microprocessors, microcomputers,
etc. The processors 120 and 170 may be implemented by hardware,
firmware, software, or a combination thereof.
If the embodiments of the present invention are implemented by
hardware, Application Specific Integrated Circuits (ASICs), Digital
Signal Processors (DSPs), Digital Signal Processing Devices
(DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate
Arrays (FPGAs), etc. may be included in the processors 120 and
170.
The present invention can also be embodied as computer-readable
code on a computer-readable recording medium. The computer-readable
recording medium includes all data storage devices that can store
data which can be thereafter read by a computer system. Examples of
the computer-readable recording medium include read-only memory
(ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy
disks and optical data storage devices. The computer-readable
recording medium can also be distributed over network coupled
computer systems so that the computer readable code is stored and
executed in a distributed fashion.
It will be apparent to those skilled in the art that various
modifications and variations can be made in the present invention
without departing from the spirit or scope of the inventions. Thus,
it is intended that the present invention covers the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
INDUSTRIAL APPLICABILITY
While the methods of generating and transmitting a pilot sequence
have been described on the basis of examples applied to the IEEE
802.11 system and HEW system, the methods can be applied to various
wireless communication systems in addition to the IEEE 802.11
system and HEW system.
* * * * *